Spread-spectrum channel sounding

ABSTRACT

An improvement to a spread-spectrum code-division-multiple-access system, using a channel sounding signal from a base station (BS) to provide initial transmitter power levels for remote stations (RS). The base station has a BS-spread-spectrum transmitter for transmitting BS-spread-spectrum signals at a first frequency and a BS-spread-spectrum receiver for receiving RS-spread-spectrum signals at a second frequency. The RS-spread-spectrum signals are transmitted by the remote stations at the second frequency. The BS-spread-spectrum signals at the first frequency are outside the correlation bandwidth of the RS-spread-spectrum signals at the second frequency. The base station has a BS transmitter for transmitting a BS-channel-sounding signal at the same carrier frequency being used by the remote stations to transmit a spread-spectrum signal to the base station. The bandwidth of the BS-channel-sounding signal is much of the bandwidth of the BS-spread-spectrum signal. Each of the remote stations has an RS receiver, for receiving the BS-channel-sounding signal at the second frequency. Each RS receiver includes an RS demodulator for tracking the BS-channel-sounding signal and outputting an RS-receiver signal, and an RS-power-level circuit. In response to the RS-receiver signal, the RS-power-level circuit adjusts the initial RS-power level. The base station has an interference-reduction subsystem, located at a front end to the BS-spread-spectrum receiver. The interference-reduction subsystem reduces the BS-channel-sounding signal, transmitted from the base station, from the RS-spread-spectrum signal arriving at the base station.

RELATED PATENTS

[0001] This patent stems from a continuation application of U.S. patentapplication Ser. No. 09/908,639, and filing date of Jul. 19, 2001,entitled CHANNEL SOUNDING FOR A SPREAD-SPECTRUM SIGNAL by inventor,DONALD L. SCHILLING, which stems from a continuation application of U.S.patent application Ser. No. 09/231,015, and filing date of Jan. 14,1999, entitled SPREAD-SPECTRUM CHANNEL SOUNDING by inventor, DONALD L.SCHILLING, which issued on Jul. 31, 2001, as U.S. Pat. No. 6,269,092.The benefit of the earlier filing date of the parent patent applicationis claimed for common subject matter pursuant to 35 U.S.C. § 120.

BACKGROUND OF THE INVENTION

[0002] This invention relates to code-division-multiple-accesscommunications, and more particularly to power settings for a remotestation, when initiating communications with a base station.

DESCRIPTION OF THE RELEVANT ART

[0003] The terrestrial communications channel is typicallynon-reciprocal. If a base station 12 transmitted, as shown in FIG. 1, ata first power level P₁ and at a first frequency f₁, a first signal to aremote station 11, then the received power at the remote station 11might be P₁₁. If the remote station 11 transmitted, at the first powerlevel P₁ and at a second frequency f₂, where the second frequency f₂ isdisplaced from the first frequency f₁ by more than a correlationbandwidth, then the received power at the base station 12 might be P₁₂,which is statistically independent of the received power P₁₁ at theremote station 11. The statistical independence, or non-reciprocalnature of the terrestrial communications channels, is of major concernto users of a code-division-multiple-access (CDMA) system.

[0004] In a direct-sequence (DS) CDMA system, a remote station'sspread-spectrum signal, received at a base station, is embedded in theinterference caused by other users. Power control of the remote stationsis therefore necessary for ensuring that during communications at thebase station, the power level received from each remote station isapproximately the same as from other remote stations communicating withthe base station. Many elaborate systems exist for power control in aDS-CDMA system, where the base station determines the power levels of areceived signal and interference, processes this information andperiodically communicates to a remote station to increase or decreaseits power level.

[0005] When a remote station is about to initiate its transmission, theremote station has little information as to what power level totransmit. Some investigators have suggested to use open-loop powercontrol, in which the remote station monitors the power received fromthe base station transmitter at the first frequency f₁, and from themonitored power, the remote station sets its initial power level of itstransmitter. The remote station, however, transmits at a secondfrequency f₂ which is not within the correlation bandwidth of the firstfrequency f₁. Since the communications channel at the first frequency f₁is statistically independent from the communications channel at thesecond frequency f₂, the open-loop power control does not work, or doesnot work well.

[0006] Another approach to power control is to initiate transmitting aremote station at a low power level, and periodically increase the powerlevel of the remote station until a signal is received at the basestation. When the power level from the remote station is sufficient forthe base station to receive, then the base station sends a response tothe remote station to stop increasing the power level, unless otherwisesignaled to do so. While this approach works, it takes considerable timedelay, particularly if packet transmissions are employed. Thus, a tenmillisecond packet might last five seconds.

SUMMARY OF THE INVENTION

[0007] A general object of the invention is to permit a remote stationto have knowledge, a priori to transmitting, of a proper power level toinitiate transmission.

[0008] Another object of the invention is to measure and initiallycorrect or compensate for Doppler shift in carrier frequency caused bythe motion of the remote station.

[0009] According to the present invention, as embodied and broadlydescribed herein, an improvement is provided to a 10 spread-spectrumsystem which has a base station (BS) and a plurality of remote stations(RS). The base station has a BS-spread-spectrum transmitter and aBS-spread-spectrum receiver. The BS-spread-spectrum transmittertransmits, using radio waves, a plurality of BS-spread-spectrum signalsat a first frequency. The BS-spread-spectrum receiver receives, at asecond frequency, a plurality of RS-spread-spectrum signals from theplurality of remote stations. The plurality of BS-spread-spectrumsignals at the first frequency are outside the correlation bandwidth ofthe plurality of RS-spread-spectrum signals at the second frequency.Each of the plurality of remote stations has an RS-spread-spectrumtransmitter for transmitting an RS-spread-spectrum signal at the secondfrequency.

[0010] The improvement includes a BS transmitter and aninterference-reduction subsystem, located at the base station receiver.The BS transmitter transmits, using radio waves, a BS-channel-soundingsignal at the second frequency. The BS-channel-sounding signal has abandwidth no more than twenty per cent of the spread-spectrum bandwidthof the plurality of RS-spread-spectrum signals, and in a preferredembodiment, the BS-channel-sounding signal has a bandwidth no more thanone percent of the spread-spectrum bandwidth of the plurality ofRS-spread-spectrum signals.

[0011] At each remote station, the improvement includes anRS-power-level circuit and an RS receiver which has an RS demodulator.The improvement at the remote station also may include afrequency-adjust circuit. The RS receiver receives theBS-channel-sounding signal at the second frequency. The RS demodulatortracks the BS-channel-sounding signal, and outputs an RS-receiversignal. Using the receiver power level of the RS-receiver signal, theRS-power-level circuit adjusts an RS-power level of theRS-spread-spectrum transmitter located at the remote station. If thefrequency-adjust circuit were employed, then the frequency-adjustcircuit, using the received RS-receiver signal as a reference,compensates to the first frequency the RS-spread-spectrum signal of theRS-spread-spectrum transmitter located at the remote station.

[0012] The interference-reduction subsystem is located at the basestation and at a front end to the BS-spread-spectrum receiver. Theinterference-reduction subsystem reduces, at the second frequency, theBS-channel-sounding signal from the plurality of RS-spread-spectrumsignals arriving at the base station.

[0013] Additional objects and advantages of the invention are set forthin part in the description which follows, and in part are obvious fromthe description, or may be learned by practice of the invention. Theobjects and advantages of the invention also may be realized andattained by means of the instrumentalities and combinations particularlypointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The accompanying drawings, which are incorporated in andconstitute a part of the specification, illustrate preferred embodimentsof the invention, and together with the description serve to explain theprinciples of the invention.

[0015]FIG. 1 illustrates a prior art remote station communicating with abase station (BS);

[0016]FIG. 2 illustrates a remote station communicating with a basestation, with channel sounding;

[0017]FIG. 3 is a block diagram illustrating a base station signal withchannel sounding added to a base station spread-spectrum transmitter;

[0018]FIG. 4 is a block diagram illustrating the improvement to theremote station spread-spectrum receiver for the BS-channel-soundingsignal;

[0019]FIG. 5 is a block diagram illustrating the improvement to theremote station spread-spectrum transmitter;

[0020]FIG. 6 is a block diagram showing an interference-reductionsubsystem at a front end to a base station spread-spectrum receiver; and

[0021]FIG. 7 is a timing diagram of how several base stations mighttransmit the BS-channel-sounding signal.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] Reference now is made in detail to the present preferredembodiments of the invention, examples of which are illustrated in theaccompanying drawings, wherein like reference numerals indicate likeelements throughout the several views.

[0023] The instant invention disclosed herein provides a novelimprovement and method to a spread-spectrum system, and moreparticularly to a cellular structure or environment with each cellcontaining a base station communicating with a plurality of remotestations, using spread-spectrum modulation. A remote station might be ahand-held unit or telephone, a connection to a computer or other modem,or other device which may be stationery or in motion.

[0024] The base station is assumed to transmit to the plurality ofremote stations at a first frequency. A particular channel from the basestation to a remote station is defined or determined by a particularchip-sequence signal, as is well known in the art for direct-sequence(DS) code-division-multiple access (CDMA) systems. The plurality ofremote stations are assumed to transmit to the base station at a secondcarrier frequency. A particular channel from a particular remote stationto the base station is defined or determined by a particularchip-sequence signal, as is well known in CDMA systems.

[0025] The invention overcomes a major problem with a plurality ofremote stations transmitting to a common base station. The plurality ofremote stations may be located at different distances, and each remotestation may have a different propagation path, to the base station.Thus, even if all the remote stations transmitted with the same powerlever, then the spread-spectrum signal from each remote station mayarrive at the base station with a different power level. A strong powerlevel from one remote station may cause sufficient interference to blockor inhibit reception of the spread-spectrum signal from a more distantremote station. This power problem is commonly known as the “near-far”problem, or power control problem: How does the spread-spectrum systemcontrol the power transmitted from each remote station, so that thepower received at the base station from each remote station isapproximately the same? If the average power received at the basestation is the same for each remote station, then the capacity islimited by the number of remote stations transmitting to the basestation. If, however, a particular remote station is sufficiently closeto the base station, and its transmitter power can block reception ofother remote stations, then capacity may be limited severely to only theremote station closest to the base station.

[0026] The invention overcomes the power control problem by permitting aremote station to have knowledge, a priori to transmitting, of a properpower level to initiate transmission. After the initial power level isused, closed-loop power control, which is well-known in the art, can beemployed.

[0027] An additional or alternative benefit from the invention is moreaccurate frequency control at a remote station. The carrier frequencytransmitted from a remote station may be shifted at the base station dueto Doppler shift in carrier frequency caused by motion. This inventioninitially corrects or compensates for Doppler shift in carrier frequencycaused by the effective motion of the remote station. The remote stationcould be at a fixed location, and the Doppler shift in carrier frequencycould be caused by time changes in the propagation path, such as treesblowing in the wind. After initial communications, a Costas loop orother frequency controlling circuit may be employed to control orcompensate for frequency changes. Such devices or circuits arewell-known in the art.

[0028] The invention broadly provides an improvement to aspread-spectrum system which has a base station (BS) and a plurality ofremote stations (RS). The base station has a BS-spread-spectrumtransmitter and a BS-spread-spectrum receiver. The BS-spread-spectrumtransmitter transmits, using radio waves, a plurality ofBS-spread-spectrum signals at a first frequency. The BS-spread-spectrumreceiver receives, at a second frequency, as radio waves, a plurality ofRS-spread-spectrum signals from the plurality of remote stations. Theplurality of BS-spread-spectrum signals at the first frequency areoutside the correlation bandwidth of the plurality of RS-spread-spectrumsignals at the second frequency. Each of the plurality of remotestations has an RS-spread-spectrum transmitter for transmitting, as aradio wave, an RS-spread-spectrum signal at the second frequency.

[0029] At the base station, the improvement includes BS-transmittermeans, and interference-reduction means. The BS-transmitter meanstransmits, as a radio wave, a BS-channel-sounding signal at the secondfrequency. The BS-channel-sounding signal has a bandwidth which is nomore than twenty per cent of the spread-spectrum bandwidth of theplurality of RS-spread-spectrum signals, and preferably not more thanone per cent of the spread-spectrum bandwidth.

[0030] The interference-reduction means is located at a front end to theBS-spread-spectrum receiver. The interference-reduction means reduces,by cancelling and notch filtering, at the second frequency, theBS-channel-sounding signal from the plurality of RS-spread-spectrumsignals arriving at the base station.

[0031] While the BS-channel-sounding signal should have a bandwidth ofno more than one per cent of the spread-spectrum bandwidth of theRS-spread-spectrum signal, system performance improves significantly asthe bandwidth of the BS-channel-sounding signal decreases. Preferably,the BS-channel-sounding signal has a bandwidth of no more than one percent, and should not exceed twenty per cent, of the spread-spectrumbandwidth of the RS-spread-spectrum signal. The BS-channel-soundingsignal may be a continuous wave signal, also known as a carrier signal.Alternatively, the BS-channel-sounding signal may be modulated withamplitude modulation (AM), frequency modulation (FM), phase modulation(PM), or a combination thereof. Amplitude-shift-keying (ASK) modulation,frequency-shift-keying (FSK) modulation or phase-shift-keying (PSK)modulation may be employed. Similarly, a narrowband spread-spectrumsignal could modulate the BS-channel-sounding signal. A combination ofthese modulations also could be employed. The modulation in theBS-channel-sounding signal can be used for base station identification,as well as for other information such as commercials, stock quotes, etc.

[0032] The bandwidth of the BS-channel-sounding signal is adeterminative factor, and a design choice, since increased bandwidthwill cause increased interference to the plurality of RS-spread-spectrumsignals, which are at the same frequency as the BS-channel-soundingsignal. At a one per cent bandwidth of the plurality ofRS-spread-spectrum signals, little degradation in system performanceresults.

[0033] Each of the plurality of remote stations includes RS-receivermeans, RS-power means and compensating means. The RS-receiving meansreceives the BS-channel-sounding signal at the second frequency, anddemodulates the BS-channel-sounding signal, and outputs an RS-receiversignal. RS-power-level means, in response to the received power level ofthe BS-channel-sounding signal, adjusts an initial RS-power level of theRS-spread-spectrum transmitter located at the remote station. Inresponse to the RS-receiver signal, the compensating means compensatesthe second frequency, for Doppler shift, of the RS-spread-spectrumsignal of the RS-spread-spectrum transmitter located at the remotestation. For example, if the carrier frequency of the receivedBS-channel-sounding signal had a negative Doppler shift from its carrierfrequency, as received at the remote station, then the compensatingmeans would impose a positive shift from the designated carrierfrequency on the transmitted RA-spread-spectrum signal. Due to motion ofthe remote station or propagation path motions in the communicationschannel, the RS-spread-spectrum signal arrives at the base station atthe corrected carrier frequency, i.e., at the second frequency. In apreferred embodiment, the RS-power means is employed to initially setthe transmitter power of the remote station. The compensating means mayalso be used to correct the transmitter frequency of theRS-spread-spectrum transmitter.

[0034] In the exemplary arrangement shown in FIG. 2, the base station 12is shown communicating, using radio waves, with a remote station withfrequency compensation. Since the BS-channel-sounding signal istransmitted, as a radio wave, from the base station 12 at the secondfrequency f₂ to the remote station 11, and the remote station 11 knowsat what frequency the BS-channel-sounding signal is suppose to bereceived, then remote station 11 can determine the Doppler frequencyshift f_(D) and compensate its transmitter frequency by a similar amountso that the RS-spread-spectrum signal arrives at the base station 12with a carrier frequency at the correct second frequency f₂. Thus, theRS-spread-spectrum signal is detected at the base station at the secondfrequency f₂, without a Doppler shift in carrier frequency f_(D). Ifmotion of the remote station caused a positive shift in the Dopplerfrequency f_(D), then the correct compensation would be to subtract theDoppler shift in carrier frequency f_(D) and transmit at frequencyf₂-f_(D). The remote station 11 also can measure the power level of theBS-channel-sounding signal, and from this measurement, set its initialpower level for transmitting the RS-spread-spectrum signal at frequencyf₂.

[0035] In FIG. 3, the improvement to the BS-spread-spectrum transmitteris shown. The signal source 29 generates the BS-channel-sounding signal.The BS-channel-sounding signal is combined in combiner 24 with theBS-spread-spectrum signal. The BS-spread-spectrum signal may begenerated, as is well known in the art, by a plurality of productdevices 16, 17, 18, which multiply a plurality of data signals d₁(t),d₂(t), . . . d_(k)(t), by a plurality of chip-sequence signals g₁(t),g₂(t), . . . g_(k)(t). The outputs from the plurality of product devices16, 17, 18 is a plurality of spread-data signals. Typically, theplurality of spread-data signals is combined linearly by a combiner 21,to generate a combined-spread-data signal. The combined-spread-datasignal is multiplied by in-phase product device 22 by a cosine signal atthe first frequency f₁, and by a quadrature-phase product device 23 by asine signal at the first frequency f₁, to generate in-phase andquadrature-phase components of the BS-spread-spectrum signal. Thein-phase component of the BS-spread-spectrum signal and thequadrature-phase component of the BS-spread-spectrum signal are thencombined to make the BS-spread-spectrum signal which is transmitted fromthe base station 12 at the first frequency, by antenna 25. Techniquesfor generating spread-spectrum signals are well known in the art, andthe technique shown in FIG. 3 is only representative.

[0036] The design of spread-spectrum transmitters is well-known in theart. Typically, the BS-spread-spectrum transmitter would be implementedin a digital signature processor (DSP) or application specificintegrated circuit (ASIC). Alternative techniques for building aspread-spectrum transmitter include using a digital matched filter, withthe matched filter having an impulse response mated to the specificchip-sequence signal desired for a spread-spectrum channel.Alternatively, surface acoustic wave (SAW) devices can be used as amatched filter. Further, the plurality of chip-sequence signals can bestored in a memory, and each time a particular digital signal is appliedto a memory address, and particular chip-sequence signal is outputted tothe combiner 21. All these techniques, and others, are well known in theart for generating spread-spectrum signals.

[0037] The signal source 29 generates the BS-channel-sounding signal,which may be a simple continuous wave signal, or a signal modulated withAM modulation, FM modulation, PM modulation, ASK modulation, FSKmodulation, PSK modulation or spread-spectrum modulation. Withmodulation, the BS-channel-sounding signal can carry data, such assignaling data or order wire data. Alternatively, theBS-channel-sounding signal can broadcast to the plurality of remotestations general information such as timing, advertisements orcommercials, and other information to update the remote station from thebase station.

[0038]FIG. 4 illustratively shows the improvement to the remote stationreceiver. The RS-spread-spectrum receiver includes the spread-spectrumreceiver 31, the RF filter 32, and the low noise amplifier (LNA) 30,coupled to the antenna 33. The RF filter 32 is coupled between thespread-spectrum receiver 21 and the low noise amplifier 30. Thecomponents for the RS-spread-spectrum receiver are well known in theart. For example, the RS-spread-spectrum receiver may be embodied as aplurality of product devices and a chip-signal generator, with outputlowpass or bandpass filters. The operation of multiplying a receivedspread-spectrum signal by a plurality of chip-sequence signals is wellknown, and can be found in many textbooks on the subject. Alternatively,the RS-spread-spectrum receiver may be embodied as a plurality ofmatched filters, which have a plurality of impulse responses matched tothe plurality of chip-sequence signals embedded in the receivedBS-spread-spectrum signal. The RS-spread-spectrum receiver may beimplemented as an integrated circuit, ASIC, SAW device, and may operateat baseband, intermediate frequency or other processing frequency.

[0039] The improvement includes RS-receiver means, which is embodied asdemodulator 35. The demodulator 35 is coupled to low noise amplifier 30,for receiving the BS-channel-sounding signal at the second frequency f₂,or at the second frequency plus or minus a Doppler shift f_(d) incarrier frequency from the second frequency f₂. The demodulator 35 mayinclude a tracking filter, phase-locked-loop (PLL) circuit, FM or PMdiscriminator, spread-spectrum receiver, or other circuitry fordemodulating the BS-channel-sounding signal. The demodulator 35demodulates the BS-channel-sounding signal, and outputs an RS-receiversignal. The RS-receiver signal is a demodulated version of the receivedBS-channel-sounding signal, and may include a power level proportionalto a received power level of the BS-channel-sounding signal, and afrequency representation or shift, of the received BS-channel-soundingsignal.

[0040] The compensating means is embodied as a frequency-adjust circuit34, coupled to the RS demodulator 35. In response to the RS-receiversignal, the frequency-adjust circuit 34 compensates to the firstfrequency the RS-spread-spectrum signal of the RS-spread-spectrumtransmitter located at the remote station. The frequency-adjust circuit34 also can provide Doppler information, including Doppler shift incarrier frequency f_(D), by way of a Doppler signal to thespread-spectrum receiver 31. The frequency-adjust circuit 34 mightinclude a local oscillator or other signal source, and a comparatorcircuit. The local oscillator or signal source from theRS-spread-spectrum transmitter generates a local signal at the secondfrequency f₂. The comparator compares the local signal with the receivedBS-channel-sounding signal, or the RS-receiver signal, to determineDoppler shift in carrier frequency f_(D). A signal with the Dopplershift in carrier frequency can be used to adjust the transmitterfrequency of the RS-spread-spectrum signal. Electronic circuits for thecomparator and frequency-adjust circuit 34, are well known in the art.The spread-spectrum receiver can adjust its oscillator circuit, or thefrequency-adjust circuit 34 can adjust the frequency of the oscillatorfor the spread-spectrum receiver 31, thereby compensating for Dopplershift in carrier frequency f_(D) due to motion.

[0041] The RS-power means may be embodied as a power-adjust circuit 36,which is coupled to the output of the demodulator 35. As shown in FIG.5, the power-adjust circuit 36 couples to a variable power amplifier 40of the RS-spread-spectrum transmitter. Depending on the power level fromthe BS-channel-sounding signal, or RS-receiver signal, the power-adjustcircuit 36 can adjust the output power of the variable power amplifier40 to a desired level. An equivalent circuit for the variable poweramplifier 40 would be a variable attenuator, which attenuates inresponse to a power-adjust signal.

[0042] Similarly, the frequency-adjust circuit 34 may couple to thesignal source 39 of the RS-spread-spectrum transmitter.

[0043] The frequency-adjust circuit 34 can offset the transmitterfrequency by the Doppler frequency f_(D), so that the RS-spread-spectrumsignal arrives at the base station 12 at the correct second frequencyf₂. By subtracting the Doppler frequency f_(D) from the second frequencyf₂, the transmitter frequency of the RS-spread-spectrum signal shiftsback to the second frequency f₂ due to the Doppler frequency added tothe carrier frequency of the RS-spread-spectrum signal, due to motion ofthe remote station.

[0044] In FIG. 5, the RS-spread-spectrum transmitter includes a productdevice 37 for multiplying a chip-sequence signal by data to generate aspread-data signal. For a positive Doppler shift in carrier frequency ofthe BS-channel-sounding signal, the spread-data signal is shifted to acarrier frequency of f₂-f_(D), by product device 38, to generate theRS-spread-spectrum signal. For a negative Doppler shift in carrierfrequency of the BS-channel-sounding signal, the spread-data signal isshifted to a carrier frequency of f₂+f_(D), by product device 38, togenerate the RS-spread-spectrum signal. The RS-spread-spectrum signal isamplified by amplifier 40 and radiated by antenna 41.

[0045]FIG. 6 shows the improvement to the BS receiver. The BStransmitter 24 is connected to a coupler 49, such as a circulator, whichconnects to the antenna 25. The antenna 25 is used in FIG. 6 fortransmitting and receiving at the frequency f₂. The RS-spread-spectrumsignals received from the remote stations pass through the coupler 49,through the bandpass filter 47 and in to the interference canceller 51.The BS-channel-sounding signal from the signal source 29 passes in tothe phase-shift attenuator 50. An output of the interference canceller51 is coupled to an input of the phase-shift attenuator 50. The signalfrom the interference canceller 51 adjusts the phase-shift attenuator 50SO as to minimize the BS-channel-sounding signal level fed in to thebase station spread-spectrum receiver 46.

[0046] As an option, a notch filter 48 may be coupled between theinterference canceller 51 and the base station spread-spectrum receiver46. The notch filter 48 notch filters the interference from theBS-channel-sounding signal. An interference canceller 51 with aphase-shift attenuator 50 for reducing interference, in general, is wellknown in the art.

[0047] The interference canceller 51 and the phase-shift attenuator 50operate in a feedback loop so as to minimize the effect of a receivedsignal at the second frequency f₂ by effectively feeding a signal fromsignal source at the second frequency f₂, 180° out of phase with thereceived signal.

[0048] The present invention may be used in a cellular architecturehaving a plurality of base stations. The BS-channel-sounding signal maybe modulated to identify a particular base station. Thus, a remotestation knows with which cell it is in communication by the modulationon the BS-channel-sounding signal. An alternative may have a pluralityof base stations, which cover a large geographic area, transmit theirrespective BS-channel-sounding signal in a respective time slot, asshown in FIG. 7. During a particular time slot, a packet may betransmitted by the respective base station. The packet may include noinformation, or identifying information. From the packet, a remotestation can determine relative power, and Doppler shift in carrierfrequency f_(D), of the particular base station to the remote unit.

[0049] The present invention also includes a method for improving aspread-spectrum system. The spread-spectrum system has at least one basestation and a plurality of remote stations (RS). The base station (BS)has a BS-spread-spectrum transmitter for transmitting, as radio waves, aplurality of BS-spread-spectrum signals at a first frequency. The basestation also has a BS-spread-spectrum receiver for receiving, at asecond frequency, a plurality of RS-spread-spectrum signals from theplurality of remote stations. The plurality of BS-spread-spectrumsignals are assumed to be at the first frequency outside a correlationbandwidth of the plurality of RS-spread-spectrum signals at the secondfrequency. Each of the plurality of remote stations has anRS-spread-spectrum transmitter for transmitting an RS-spread-spectrumsignal at the second frequency. The method comprises the steps oftransmitting, from a BS transmitter, located at the base station, aBS-channel-sounding signal at the second frequency. TheBS-channel-sounding signal has a bandwidth no more than twenty per centof the spread-spectrum bandwidth of the plurality of RS-spread-spectrumsignals, and preferably less than one percent of the spread-spectrumbandwidth of the plurality of RS-spread-spectrum signals. The methodincludes receiving, at each of the plurality of remote stations with anRS receiver, the BS-channel-sounding signal at the second frequency, andreceiving, at each of the plurality of remote stations with an RSdemodulator, a the BS-channel-sounding signal. The RS demodulatoroutputs an RS-receiver signal. The method further includes the step ofcompensating, in response to the RS-receiver signal, a frequency-adjustcircuit to the first frequency the RS-spread-spectrum signal of theRS-spread-spectrum transmitter located at the remote station. The methodmay adjust, in response to the RS-receiver signal, an initial RS-powerlevel of the RS-spread-spectrum transmitter located at the remotestation. At the base station, the method includes the step of reducing,at the second frequency, the BS-channel-sounding signal from theRS-spread-spectrum signal arriving at the base station.

[0050] The method optionally may further include the step ofcompensating, in response to RS-receiver signal, to the first frequencythe RS-spread-spectrum signal of the RS-spread-spectrum transmitterlocated at the remote station.

[0051] It will be apparent to those skilled in the art that variousmodifications can be made to the spread-spectrum channel soundingimprovement of the instant invention without departing from the scope orspirit of the invention, and it is intended that the present inventioncover modifications and variations of the spread-spectrum channelsounding improvement provided they come within the scope of the appendedclaims and their equivalents.

I claim:
 1. An improvement to a spread-spectrum system having a basestation and a plurality of remote stations (RS), with said base station(BS) having a BS-spread-spectrum transmitter for transmitting aplurality of BS-spread-spectrum signals at a first frequency and aBS-spread-spectrum receiver for receiving, at a second frequency, aplurality of RS-spread-spectrum signals from said plurality of remotestations, with the plurality of BS-spread-spectrum signals at the firstfrequency outside a correlation bandwidth of the plurality ofRS-spread-spectrum signals at the second frequency, with each of saidplurality of remote stations having an RS-spread-spectrum transmitterfor transmitting an RS-spread-spectrum signal at the second frequency,the improvement comprising: a BS transmitter, located at said basestation, for transmitting a BS-channel-sounding signal at the secondfrequency, with the BS-channel-sounding signal having a bandwidth nomore than twenty per cent of the spread-spectrum bandwidth of theplurality of RS-spread-spectrum signals; each of said plurality ofremote stations including an RS receiver, for receiving theBS-channel-sounding signal at the second frequency, each RS receiverhaving, an RS demodulator for tracking the BS-channel-sounding signal,thereby outputting an RS-receiver signal; a frequency-adjust circuit,coupled to said RS demodulator and responsive to the RS-receiver signal,for compensating to the second frequency the RS-spread-spectrum signalof said RS-spread-spectrum transmitter located at said remote station;each of said plurality of remote stations including an RS-power-levelcircuit, responsive to the RS-receiver signal, for adjusting an initialRS-power level of said RS-spread-spectrum transmitter located at saidremote station; and an interference-reduction subsystem, located at saidbase station and at a front end to said BS-spread-spectrum receiver, forreducing, at the second frequency, the BS-channel-sounding signal fromthe RS-spread-spectrum signal arriving at said base station.
 2. Animprovement to a spread-spectrum system having a base station and aplurality of remote stations (RS), with said base station (BS) having aBS-spread-spectrum transmitter for transmitting a plurality ofBS-spread-spectrum signals at a first frequency and a BS-spread-spectrumreceiver for receiving, at a second frequency, a plurality ofRS-spread-spectrum signals from said plurality of remote stations, withthe plurality of BS-spread-spectrum signals at the first frequencyoutside a correlation bandwidth of the plurality of RS-spread-spectrumsignals at the second frequency, with each of said plurality of remotestations having an RS-spread-spectrum transmitter for transmitting anRS-spread-spectrum signal at the second frequency, the improvementcomprising: a BS transmitter, located at said base station, fortransmitting, using radio waves, a BS-channel-sounding signal at thesecond frequency, with the BS-channel-sounding signal having a bandwidthno more than twenty per cent of the spread-spectrum bandwidth of theplurality of RS-spread-spectrum signals; each of said plurality ofremote stations including an RS receiver, for receiving theBS-channel-sounding signal at the second frequency, each RS receiverhaving an RS demodulator for tracking the BS-channel-sounding signal,thereby outputting an RS-receiver signal; each of said plurality ofremote stations including an RS-power-level circuit, responsive to theRS-receiver signal, for adjusting an initial RS-power level of saidRS-spread-spectrum transmitter located at said remote station; and aninterference-reduction subsystem, located at said base station and at afront end to said BS-spread-spectrum receiver, for reducing theBS-channel-sounding signal from the RS-spread-spectrum signal arrivingat said base station.
 3. An improvement to a spread-spectrum systemhaving a base station and a plurality of remote stations (RS), with saidbase station (BS) having a BS-spread-spectrum transmitter fortransmitting a plurality of BS-spread-spectrum signals at a firstfrequency and a BS-spread-spectrum receiver for receiving, at a secondfrequency, a plurality of RS-spread-spectrum signals from said pluralityof remote stations, with the plurality of BS-spread-spectrum signals atthe first frequency outside a correlation bandwidth of the plurality ofRS-spread-spectrum signals at the second frequency, with each of saidplurality of remote stations having an RS-spread-spectrum transmitterfor transmitting an RS-spread-spectrum signal at the second frequency,the improvement comprising: a BS transmitter, located at said basestation, for transmitting, using radio waves, a BS-channel-soundingsignal at the second frequency, with the BS-channel-sounding signalhaving a bandwidth no more than twenty per cent of the spread-spectrumbandwidth of the plurality of RS-spread-spectrum signals; each of saidplurality of remote stations including an RS receiver, for receiving theBS-channel-sounding signal at the second frequency, each RS receiverhaving, an RS demodulator for tracking the BS-channel-sounding signal,thereby outputting an RS-receiver signal; a frequency-adjust circuit,coupled to said RS demodulator and responsive to the RS-receiver signal,for compensating to the second frequency the RS-spread-spectrum signalof said RS-spread-spectrum transmitter located at said remote station;and an interference-reduction subsystem, located at said base stationand at a front end to said BS-spread-spectrum receiver, for reducing theBS-channel-sounding signal from the RS-spread-spectrum signal arrivingat said base station.
 4. The improvement to the spread-spectrum systemas set forth in claim 1, 2 or 3, with said BS transmitter transmittingthe BS-channel-sounding signal at the second frequency, with theBS-channel-sounding signal having a bandwidth no more than ten per centof the spread-spectrum bandwidth of the plurality of RS-spread-spectrumsignals.
 5. The improvement to the spread-spectrum system as set forthin claim 1, 2 or 3, with said BS transmitter transmitting theBS-channel-sounding signal at the second frequency, with theBS-channel-sounding signal having a bandwidth no more than five per centof the spread-spectrum bandwidth of the plurality of RS-spread-spectrumsignals.
 6. The improvement to the spread-spectrum system as set forthin claim 1, 2 or 3, with said BS transmitter transmitting theBS-channel-sounding signal at the second frequency, with theBS-channel-sounding signal having a bandwidth no more than one per centof the spread-spectrum bandwidth of the plurality of RS-spread-spectrumsignals.
 7. An improvement to a spread-spectrum system having a basestation and a plurality of remote stations (RS), with said base station(BS) having a BS-spread-spectrum transmitter for transmitting aplurality of BS-spread-spectrum signals at a first frequency and aBS-spread-spectrum receiver for receiving, at a second frequency, aplurality of RS-spread-spectrum signals from said plurality of remotestations, with the plurality of BS-spread-spectrum signals at the firstfrequency outside a correlation bandwidth of the plurality ofRS-spread-spectrum signals at the second frequency, with each of saidplurality of remote stations having an RS-spread-spectrum transmitterfor transmitting an RS-spread-spectrum signal at the second frequency,the improvement comprising: BS-transmitter means, located at said basestation, for transmitting, using radio waves, a BS-channel-soundingsignal at the second frequency, with the BS-channel-sounding signalhaving a bandwidth no more than twenty per cent of the spread-spectrumbandwidth of the plurality of RS-spread-spectrum signals; each of saidplurality of remote stations including RS-receiver means, for receivingthe BS-channel-sounding signal at the second frequency, and for trackingthe BS-channel-sounding signal, thereby outputting an RS-receiversignal; each of said plurality of remote stations includingRS-power-level means, responsive to the RS-receiver signal, foradjusting an initial RS-power level of said RS-spread-spectrumtransmitter located at said remote station; and interference-reductionmeans, located at said base station and at a front end to saidBS-spread-spectrum receiver, for reducing the BS-channel-sounding signalfrom the RS-spread-spectrum signal arriving at said base station.
 8. Theimprovement to the spread-spectrum system as set forth in claim 7, withsaid RS-receiver means at each of said plurality of remote stationsfurther including compensating means, responsive to RS-receiver signal,for compensating to the second frequency the RS-spread-spectrum signalof said RS-spread-spectrum transmitter located at said remote station.9. The improvement to the spread-spectrum system as set forth in claim 7or 8, with said BS transmitter transmitting the BS-channel-soundingsignal at the second frequency, with the BS-channel-sounding signalhaving a bandwidth no more than ten per cent of the spread-spectrumbandwidth of the plurality of RS-spread-spectrum signals.
 10. Theimprovement to the spread-spectrum system as set forth in claim 7 or 8,with said BS transmitter transmitting the BS-channel-sounding signal atthe second frequency, with the BS-channel-sounding signal having abandwidth no more than five per cent of the spread-spectrum bandwidth ofthe plurality of RS-spread-spectrum signals.
 11. The improvement to thespread-spectrum system as set forth in claim 7 or 8, with said BStransmitter transmitting the BS-channel-sounding signal at the secondfrequency, with the BS-channel-sounding signal having a bandwidth nomore than one per cent of the spread-spectrum bandwidth of the pluralityof RS-spread-spectrum signals.
 12. A method for improving aspread-spectrum system having a base station and a plurality of remotestations (RS), with said base station (BS) having a BS-spread-spectrumtransmitter for transmitting a plurality of BS-spread-spectrum signalsat a first frequency and a BS-spread-spectrum receiver for receiving, ata second frequency, a plurality of RS-spread-spectrum signals from saidplurality of remote stations, with the plurality of BS-spread-spectrumsignals at the first frequency outside a correlation bandwidth of theplurality of RS-spread-spectrum signals at the second frequency, witheach of said plurality of remote stations having an RS-spread-spectrumtransmitter for transmitting an RS-spread-spectrum signal at the secondfrequency, the method comprising the steps of: transmitting, using radiowaves, from a BS transmitter, located at said base station, aBS-channel-sounding signal at the second frequency, with theBS-channel-sounding signal having a bandwidth no more than twenty percent of the spread-spectrum bandwidth of the plurality ofRS-spread-spectrum signals; receiving, at each of said plurality ofremote stations with an RS receiver, the BS-channel-sounding signal atthe second frequency; tracking, at each of said plurality of remotestations with an RS demodulator, a the BS-channel-sounding signal,thereby generating an RS-receiver signal; adjusting, in response to theRS-receiver signal, an initial RS-power level of said RS-spread-spectrumtransmitter located at said remote station; and reducing theBS-channel-sounding signal from the RS-spread-spectrum signal arrivingat said base station.
 13. The method for improving the spread-spectrumsystem as set forth in claim 12, further including the step ofcompensating, in response to RS-receiver signal, to the second frequencythe RS-spread-spectrum signal of said RS-spread-spectrum transmitterlocated at said remote station.
 14. The method for improving thespread-spectrum system as set forth in claim 12 or 13, with the step oftransmitting the BS-channel-sounding signal at the second frequency,including the step of transmitting the BS-channel-sounding signal with abandwidth no more than ten per cent of the spread-spectrum bandwidth ofthe plurality of RS-spread-spectrum signals.
 15. The method forimproving the spread-spectrum system as set forth in claim 12 or 13,with the step of transmitting the BS-channel-sounding signal at thesecond frequency, including the step of transmitting theBS-channel-sounding signal with a bandwidth no more than one per cent ofthe spread-spectrum bandwidth of the plurality of RS-spread-spectrumsignals.
 16. The method for improving the spread-spectrum system as setforth in claim 12 or 13, with the step of transmitting theBS-channel-sounding signal at the second frequency, including the stepof transmitting the BS-channel-sounding signal with a bandwidth no morethan one per cent of the spread-spectrum bandwidth of the plurality ofRS-spread-spectrum signals.
 17. The method for improving thespread-spectrum system as set forth in claim 12 or 13, with the step ofreducing the BS-channel-sounding signal further including the step ofnotch filtering the BS-channel-sounding signal from the plurality ofRS-spread-spectrum signals.
 18. The method for improving thespread-spectrum system as set forth in claim 14, with the step ofreducing the BS-channel-sounding signal further including the step ofnotch filtering the BS-channel-sounding signal from the plurality ofRS-spread-spectrum signals.
 19. The method for improving thespread-spectrum system as set forth in claim 15, with the step ofreducing the BS-channel-sounding signal further including the step ofnotch filtering the BS-channel-sounding signal from the plurality ofRS-spread-spectrum signals.
 20. The method for improving thespread-spectrum system as set forth in claim 16, with the step ofreducing the BS-channel-sounding signal further including the step ofnotch filtering the BS-channel-sounding signal from the plurality ofRS-spread-spectrum signals.
 21. An improvement to a spread-spectrumsystem having a plurality of base stations covering a geographic area,with each base station communicating within a geographic cell with aplurality of remote stations (RS), with each base station (BS) having aBS-spread-spectrum transmitter for transmitting a plurality ofBS-spread-spectrum signals at a first frequency and a BS-spread-spectrumreceiver for receiving, at a second frequency, a plurality ofRS-spread-spectrum signals from said plurality of remote stations, withthe plurality of BS-spread-spectrum signals at the first frequencyoutside a correlation bandwidth of the plurality of RS-spread-spectrumsignals at the second frequency, with each of said plurality of remotestations having an RS-spread-spectrum transmitter for transmitting anRS-spread-spectrum signal at the second frequency, the improvementcomprising: a BS transmitter, located at each base station, fortransmitting a BS-channel-sounding signal at the second frequency, withthe BS-channel-sounding signal transmitted within a respective time slotassigned to the respective BS transmitter, and having a bandwidth nomore than twenty per cent of the spread-spectrum bandwidth of theplurality of RS-spread-spectrum signals; each of said plurality ofremote stations including an RS receiver, for receiving theBS-channel-sounding signal at the second frequency, each RS receiverhaving, an RS demodulator for tracking the BS-channel-sounding signal,thereby outputting an RS-receiver signal; a frequency-adjust circuit,coupled to said RS demodulator and responsive to the RS-receiver signal,for compensating to the second frequency the RS-spread-spectrum signalof said RS-spread-spectrum transmitter located at said remote station;each of said plurality of remote stations including an RS-power-levelcircuit, responsive to the RS-receiver signal, for adjusting an initialRS-power level of said RS-spread-spectrum transmitter located at saidremote station; and an interference-reduction subsystem, located at saidbase station and at a front end to said BS-spread-spectrum receiver, forreducing, at the second frequency, the BS-channel-sounding signal fromthe RS-spread-spectrum signal arriving at said base station.
 22. Animprovement to a spread-spectrum system having a plurality of basestations covering a geographic area, with each base stationcommunicating within a geographic cell with a plurality of remotestations (RS), with each base station (BS) having a BS-spread-spectrumtransmitter for transmitting a plurality of BS-spread-spectrum signalsat a first frequency and a BS-spread-spectrum receiver for receiving, ata second frequency, a plurality of RS-spread-spectrum signals from saidplurality of remote stations, with the plurality of BS-spread-spectrumsignals at the first frequency outside a correlation bandwidth of theplurality of RS-spread-spectrum signals at the second frequency, witheach of said plurality of remote stations having an RS-spread-spectrumtransmitter for transmitting an RS-spread-spectrum signal at the secondfrequency, the improvement comprising: a BS transmitter, located at eachbase station, for transmitting a BS-channel-sounding signal at thesecond frequency, with the BS-channel-sounding signal transmitted withina respective time slot assigned to the respective BS transmitter, andhaving a bandwidth no more than twenty per cent of the spread-spectrumbandwidth of the plurality of RS-spread-spectrum signals; each of saidplurality of remote stations including an RS receiver, for receiving theBS-channel-sounding signal at the second frequency, each RS receiverhaving an RS demodulator for tracking the BS-channel-sounding signal,thereby outputting an RS-receiver signal; each of said plurality ofremote stations including an RS-power-level circuit, responsive to theRS-receiver signal, for adjusting an initial RS-power level of saidRS-spread-spectrum transmitter located at said remote station; and aninterference-reduction subsystem, located at said base station and at afront end to said BS-spread-spectrum receiver, for reducing, at thesecond frequency, the BS-channel-sounding signal from theRS-spread-spectrum signal arriving at said base station.
 23. Animprovement to a spread-spectrum system having a plurality of basestations covering a geographic area, with each base stationcommunicating within a geographic cell with a plurality of remotestations (RS), with each base station (BS) having a BS-spread-spectrumtransmitter for transmitting a plurality of BS-spread-spectrum signalsat a first frequency and a BS-spread-spectrum receiver for receiving, ata second frequency, a plurality of RS-spread-spectrum signals from saidplurality of remote stations, with the plurality of BS-spread-spectrumsignals at the first frequency outside a correlation bandwidth of theplurality of RS-spread-spectrum signals at the second frequency, witheach of said plurality of remote stations having an RS-spread-spectrumtransmitter for transmitting an RS-spread-spectrum signal at the secondfrequency, the improvement comprising: a BS transmitter, located at eachbase station, for transmitting a BS-channel-sounding signal at thesecond frequency, with the BS-channel-sounding signal transmitted withina respective time slot assigned to the respective BS transmitter, andhaving a bandwidth no more than twenty per cent of the spread-spectrumbandwidth of the plurality of RS-spread-spectrum signals; each of saidplurality of remote stations including an RS receiver, for receiving theBS-channel-sounding signal at the second frequency, each RS receiverhaving, an RS demodulator for tracking the BS-channel-sounding signal,thereby outputting an RS-receiver signal; a frequency-adjust circuit,coupled to said RS demodulator and responsive to the RS-receiver signal,for compensating to the second frequency the RS-spread-spectrum signalof said RS-spread-spectrum transmitter located at said remote station;and an interference-reduction subsystem, located at said base stationand at a front end to said BS-spread-spectrum receiver, for reducing, atthe second frequency, the BS-channel-sounding signal from theRS-spread-spectrum signal arriving at said base station.
 24. Theimprovement to the spread-spectrum system as set forth in claim 21, 22,or 23, with said BS transmitter transmitting the BS-channel-soundingsignal at the second frequency, with the BS-channel-sounding signalhaving a bandwidth no more than ten per cent of the spread-spectrumbandwidth of the plurality of RS-spread-spectrum signals.
 25. Theimprovement to the spread-spectrum system as set forth in claim 21, 22,or 23, with said BS transmitter transmitting the BS-channel-soundingsignal at the second frequency, with the BS-channel-sounding signalhaving a bandwidth no more than five per cent of the spread-spectrumbandwidth of the plurality of RS-spread-spectrum signals.
 26. Theimprovement to the spread-spectrum system as set forth in claim 21, 22,or 23, with said BS transmitter transmitting the BS-channel-soundingsignal at the second frequency, with the BS-channel-sounding signalhaving a bandwidth no more than one per cent of the spread-spectrumbandwidth of the plurality of RS-spread-spectrum signals.
 27. The methodfor improving the spread-spectrum system as set forth in claim 1, 2 or3, with said interference-reduction subsystem including a notch filterfor notch filtering the BS-channel-sounding signal from the plurality ofRS-spread-spectrum signals.
 28. The method for improving thespread-spectrum system as set forth in claim 4, with saidinterference-reduction subsystem including a notch filter for notchfiltering the BS-channel-sounding signal from the plurality ofRS-spread-spectrum signals.
 29. The method for improving thespread-spectrum system as set forth in claim 5, with saidinterference-reduction subsystem including a notch filter for notchfiltering the BS-channel-sounding signal from the plurality ofRS-spread-spectrum signals.
 30. The method for improving thespread-spectrum system as set forth in claim 6, with saidinterference-reduction subsystem including a notch filter for notchfiltering the BS-channel-sounding signal from the plurality ofRS-spread-spectrum signals.
 31. The method for improving thespread-spectrum system as set forth in claim 7 or 8, with saidinterference-reduction means including a notch filter for notchfiltering the BS-channel-sounding signal from the plurality ofRS-spread-spectrum signals.
 32. The method for improving thespread-spectrum system as set forth in claim 9, with saidinterference-reduction means including a notch filter for notchfiltering the BS-channel-sounding signal from the plurality ofRS-spread-spectrum signals.
 33. The method for improving thespread-spectrum system as set forth in claim 10, with saidinterference-reduction means including a notch filter for notchfiltering the BS-channel-sounding signal from the plurality ofRS-spread-spectrum signals.
 34. The method for improving thespread-spectrum system as set forth in claim 11, with saidinterference-reduction means including a notch filter for notchfiltering the BS-channel-sounding signal from the plurality ofRS-spread-spectrum signals.